Natural killer cells are a subpopulation of Fc receptor positive, nonadherent, nonphagocytic cytotoxic effector cells present in the large granular lymphocyte fraction of human peripheral blood lyumphocytes which can recognize and lyse tumor cells, virally infected cells, fetal fibroblast, thymus cells and B cell lines and may be involved in auto-regulation of at least thymocytes, bone marrow stems cells, epithelial cells, and terminally differentiated B cells. The long term objective of this study is to determine the molecular events which occur during the lethal hit stage of natural killer cell-mediated cytotoxicity (NK CMC). The objective of this present proposal is to determine the relevance of lytic factors secreted from NK cells to events occurring during the lethal hit stage of direct NK CMC. Since it has been shown that the activity of natural killer cell-derived cytolytic factor (NKCF) is closely correlated to that which occurs during lysis of programmed target cells, this study will examine in detail, the role that these factors may play in direct NK CMC. This shall be accomplished by separating these factors and preparing monoclonal antibodies by screening for antibodies that inhibit NKCF lytic activity. These anti-NKCF antibodies will be used to determine the fate of target cell associated NKCF and where in the NKCF reaction sequence that antibodies to NKCF inhibit NKCF-mediated lysis. The role of NKCF in direct NK CMC will be determined by interacting labeled anti-NKCF antibodies with resting and activated NK cells to determine its intracellular location and to kinetically determine is location in both the NK cell and the target cell during the stages of NK CMC. These studies in conjunction with kinetic studies will allow us to determine where in the NK reaction sequence that antibodies to NKCF inhibit NK CMC and killer cell independent lysis (KCIL). These results should determine whether NKCF is a lytic factor that is involved in the terminal lethal hit phase of the NK lytic mechanism.